Add a HIR to the compiler

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+- Feature Name: N/A
+- Start Date: 2015-07-06
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+
+# Summary
+
+Add a high-level intermediate representation (HIR) to the compiler. This is
+basically a new (and additional) AST more suited for use by the compiler.
+
+This is purely an implementation detail of the compiler. It has no effect on the
+language.
+
+Note that adding a HIR does not preclude adding a MIR or LIR in the future.
+
+
+# Motivation
+
+Currently the AST is used by libsyntax for syntactic operations, by the compiler
+for pretty much everything, and in syntax extensions. I propose splitting the
+AST into a libsyntax version that is specialised for syntactic operation and
+will eventually be stabilised for use by syntax extensions and tools, and the
+HIR which is entirely internal to the compiler.
+
+The benefit of this split is that each AST can be specialised to its task and we
+can separate the interface to the compiler (the AST) from its implementation
+(the HIR). Specific changes I see that could happen are more ids and spans in
+the AST, the AST adhering more closely to the surface syntax, the HIR becoming
+more abstract (e.g., combining structs and enums), and using resolved names in
+the HIR (i.e., performing name resolution as part of the AST->HIR lowering).
+
+Not using the AST in the compiler means we can work to stabilise it for syntax
+extensions and tools: it will become part of the interface to the compiler.
+
+I also envisage all syntactic expansion of language constructs (e.g., `for`
+loops, `if let`) moving to the lowering step from AST to HIR, rather than being
+AST manipulations. That should make both error messages and tool support better
+for such constructs. It would be nice to move lifetime elision to the lowering
+step too, in order to make the HIR as explicit as possible.
+
+
+# Detailed design
+
+Initially, the HIR will be an (almost) identical copy of the AST and the
+lowering step will simply be a copy operation. Since some constructs (macros,
+`for` loops, etc.) are expanded away in libsyntax, these will not be part of the
+HIR. Tools such as the AST visitor will need to be duplicated.
+
+The compiler will be changed to use the HIR throughout (this should mostly be a
+matter of change the imports). Incrementally, I expect to move expansion of
+language constructs to the lowering step. Further in the future, the HIR should
+get more abstract and compact, and the AST should get closer to the surface
+syntax.
+
+
+# Drawbacks
+
+Potentially slower compilations and higher memory use. However, this should be
+offset in the long run by making improvements to the compiler easier by having a
+more appropriate data structure.
+
+
+# Alternatives
+
+Leave things as they are.
+
+Skip the HIR and lower straight to a MIR later in compilation. This has
+advantages which adding a HIR does not have, however, it is a far more complex
+refactoring and also misses some benefits of the HIR, notably being able to
+stabilise the AST for tools and syntax extensions without locking in the
+compiler.
+
+
+# Unresolved questions
+
+How to deal with spans and source code. We could keep the AST around and
+reference back to it from the HIR. Or we could copy span information to the HIR
+(I plan on doing this initially). Possibly some other solution like keeping the
+span info in a side table (note that we need less span info in the compiler than
+we do in libsyntax, which is in turn less than tools want).